Apparatus, system and method to reduce wafer warpage

ABSTRACT

Typically, the frontside of a wafer is protected by a tape during backgrinding. Electrostatic charge may accumulate on the tape during the backgrinding operation. The wafer may warp after the backgrinding operation because the thinned wafer is not sufficiently rigid to counteract the bending forces resulting from the accumulation of electrostatic charge. In order to reduce wafer warpage, ionized air may be directed onto the wafer and tape to reduce the accumulation of electrostatic charge.

FIELD OF THE INVENTION

This invention relates to fabrication of semiconductor devices and moreparticularly to a fabrication process of a semiconductor deviceincluding a grinding step applied to a back surface of a semiconductorsubstrate while protecting the front side thereof by an adhesive medium.

BACKGROUND OF THE INVENTION

The trend towards larger and thicker wafers presents several problems inthe packaging process. Thicker wafers require the more expensive sawthrough method at die separation. Although sawing produces a higherquality die, the process is more expensive in time and consumption ofdiamond tipped saws. A thicker die also requires deeper die attachcavities, resulting in a more expensive package. Both of theseundesirable results are avoided by thinning the wafers before dieseparation. Another reason for thinning wafers is that the wafer backsare not protected during doping operations such that the dopants formelectrical junctions in the wafer back. These electrical junctions mayinterfere with back contact conduction. As such, the wafers are thinnedto remove the electrical junctions.

Typically, the wafers are thinned to a predetermined thickness by abackside grinding process. For example, the thickness of an 8-inchdiameter wafer may be reduced from about 850 microns to about 180microns or less. In backgrinding, the frontside of the wafer may bescratched and/or the wafer may broken because the wafer is held down onthe grinder or polishing surface. In order to protect the wafer fromsuch scratches and breakage, a protective tape is applied to the frontsurface of the wafer. Generally, the protective tape comprises a tapebase and an adhesive layer. The tape base has a thickness of about 100to 150 microns and is formed of a polymer such polyolefin,polyethylense, or polyvinyl chloride. The adhesive layer is typically anacrylic resin with a thickness of 30 to 40 microns.

A typical backgrinding apparatus comprises a supporting base and atleast one grinding wheel assembly which faces the supporting base. Thesupporting base typically has a holding table, and the surface of theholding table protrudes beyond the surface of the supporting base. Thegrinding wheel assembly includes a rotatably mounted support shaft and agrinding wheel mounted to the supporting shaft. In the aforesaidbackgrinding apparatus, a wafer is placed on the surface of the holdingtable and secured by vacuum. The grinding wheel is rotated by rotatingthe supporting shaft. The surface of the wafer is ground by moving thesupporting base relative the grinding wheel assembly. After the wafer isground to the predetermined thickness, the wafer is transferred to acarrier, and the carrier is transferred to a detapping apparatus wherethe protective tape is removed from the wafer.

One of the problems resulting from the wafer processing industrymigrating to 8 inch or larger wafers is that the wafer is often toofragile for handling after the backgrinding operation, wherein the waferis likely to be broken or damaged during subsequent handling.Furthermore, stresses induced in the wafer by the grinding and polishingprocess need to be controlled to prevent wafer and die warping. Waferwarping interferes with the die separation process due to die breakage,and die warping creates die attach problems in the packaging process.

Additionally, it has been observed that an electrostatic charge mayaccumulate on the protective tape and wafer during the grindingoperation. Such accumulation of electrostatic charge warps the wafer,thereby further complicating the handling and placement of the wafers.In particular, it is often difficult to load and unload the wafers fromthe carrier and/or boat. For example, after grinding, the wafer istransferred by an arm mechanism to a carrier located at an exit station.If the wafer is severely warped, the arm mechanism may be unable to feedthe wafer into a slot of the carrier. If the arm mechanism is able tothe load the wafer into the carrier, there may be insufficient clearancefor the arm mechanism to feed a subsequent wafer into the carrier. As aresult, the arm mechanism may break an already loaded wafer during theloading process and/or break both the wafer being loaded and an alreadyloaded wafer.

Another problem associated with warpage is that the wafers may besufficiently flat such that arm mechanism is able to successfully feedall the wafers into the carrier located at the exit station. The carrieris then transferred to a detapping apparatus where the protective tapeis removed from the frontside of the wafer. However, the extent of waferwarpage is enhanced by the attractive forces acting upon adjacentwafers. For example, a wafer with a positively charged frontside will beattracted to an adjacent wafer with a negatively charged backside tocause further warpage of the adjacent wafer. The increased warpagedecreases the clearance between wafers to the extent that an armmechanism may be unable to transfer a wafer from the carrier to thedetapping apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of a carrier loaded with wafers which aresufficiently rigid to remain flat when subjected to an accumulation ofelectrostatic charge.

FIG. 1B is a plan view of a carrier loaded with wafers which are warpeddue to an accumulation of electrostatic charge.

FIG. 1C is a plan view of the wafers shown in FIG. 1B being transformedfrom a warped state to a flat state by neutralizing the accumulation ofelectrostatic charge.

FIG. 2 is a diagram illustrating a system in which one embodiment of theinvention can be practiced.

FIG. 3 is a flowchart illustrating a process for fabricating anexemplary semiconductor device in accordance with the system shown inFIG. 2.

FIG. 4 is a schematic diagram of a protective tape applying apparatus inaccordance with the system shown in FIG. 2.

FIG. 5 is a schematic diagram of a backgrinding apparatus in accordancewith the system shown in FIG. 2.

FIG. 6 is a schematic diagram of a detapping apparatus in accordancewith the system shown in FIG. 2.

FIG. 7 is a schematic diagram of a dicing tape applying apparatus inaccordance with the system shown in FIG. 2.

FIG. 8 is a schematic diagram of a wafer dicing apparatus in accordancewith the system shown in FIG. 2.

FIG. 9 is an alternative exemplary embodiment of a backgrindingapparatus in accordance with the system shown in FIG. 2.

FIG. 10 is another exemplary embodiment in which warped wafers may beflattened by neutralizing the accumulation of electrostatic charge.

DETAILED DESCRIPTION OF THE INVENTION

Detailed descriptions are provided herein. It is to be understood,however, that the present invention may be embodied in various forms.Therefore, specific details disclosed herein are not to be interpretedas limiting, but rather as a basis for the claims and as arepresentative for teaching one skilled in the art to employ theinvention in virtually any appropriately detailed system, structure ormanner.

FIG. 1A illustrates a carrier 10 loaded with wafers 12 exhibiting anaccumulation of electrostatic charge as a result of a grindingoperation. The frontside 14 of the wafers 12 have a net positive chargeand the backside 16 of the wafers 12 have a net negative charge as aresult of the grinding operation. The wafers 12 remain flat because theyare sufficiently rigid to counteract the bending forces resulting fromthe accumulation of electrostatic charge. It has been observed that8-inch wafers which are ground to a thickness of about 13 mils thick donot exhibit warpage. Of course, wafer warpage becomes more prominent fora given thickness as the diameter of the wafer increases.

FIG. 1B illustrates the carrier 10 loaded with wafers 18 which arewarped after the grinding operation due to an accumulation ofelectrostatic charge. These wafers 18 are not sufficiently rigid tocounteract the bending forces resulting from the accumulation ofelectrostatic charge. It has been observed that 8-inch wafers, which areground to a thickness of about 7 mils, exhibit sufficient warpage tonegatively affect the handling of wafers. FIG. 1C illustrates the samewafers 18 shown in FIG. 1B wherein the bottom wafer 18 is flattened byneutralizing the accumulation of electrostatic charge with ionized air.

FIG. 2 illustrates a system 20 for performing the backgrinding anddicing process, and FIG. 3 is a flowchart 22 illustrating a process forfabricating the semiconductor device in accordance with the system shownin FIG. 2. The system comprises a protective tape applying apparatus 26,a backgrinding apparatus 28, a detapping apparatus 30, a dicing tapeapplying apparatus 32, and a wafer dicing apparatus 34. In thisparticular embodiment, 8-inch wafers 36 having an initial thickness ofabout 850 microns are utilized. However, it is noted that the system maybe adapted to process any sized wafer. After a circuit pattern 38 isformed on the frontside 40 of the wafers 36, the wafers 36 are loadedonto a carrier 42 and transferred to the protective tape applyingapparatus 26 as shown in FIG. 4. The protective tape applying apparatus26 includes a loading station 44, a protective tape applying station 46,and an unloading station 48. An operator places the carrier 42 at theloading station 44. A transfer arm 50 unloads the wafer 36 from thecarrier 42 and transfers the wafer 36 to the protective tape applyingstation 46. A protective tape 52 is dispensed from a roll 54 andlaminated onto the frontside 38 of the wafer 36. A cutter 56 cuts theprotective tape 52 along the outer edge of the wafer 36, and theprotective tape 52 is pressed onto the wafer 36 by a roller 58. Atransfer arm 60 transfers the wafer 36 from the protective tape applyingstation 46 to a carrier 62 located at the unloading station 48. Theprotective tape application process is repeated until the carrier 62 isfully loaded with wafers 36.

The carrier 62 is then transferred to the backgrinding apparatus 28.Referring to FIG. 5, the backgrinding apparatus 28 includes a loadingstation 64, a precleaning station 66, a rough grinding station 68, afinish grinding station 70, a post cleaning station 72, a final cleaningstation 74, and an unloading station 76. A loader arm 78 transfers thewafer 36 from the carrier 62 to the precleaning station 66. The wafer 36is secured to a vacuum chuck table 80 such that the protective tape 52contacts the surface of the vacuum chuck table 80. That is, a backside82 of the wafer 36 faces upwardly. The vacuum chuck table 80 is rotatedand deionized water is dispensed onto the backside 82 of the wafer 36.The backside 36 of the wafer 82 is further cleaned with a Teflonscrubber 84 during the dispensing of deionized water. The wafer 36 isthen spin dried with nitrogen air.

With the wafer 36 precleaned, a transfer arm 86 transfers the wafer 36from the prelceaning station 66 to the rough grinding station 68. Thewafer 36 is secured to a vacuum chuck table 88. The size of the vacuumchuck table 88 is larger than the wafer 36, and, thus, the entiresurface of the wafer 36 is supported by the vacuum chuck table 88 andfixed on the vacuum chuck table 88 by suction. A course grinding mediais dispensed onto the wafer 36, and the thickness of the wafer 36 isreduced to a predetermined thickness by a rough grinding tool 90, suchas a diamond wheel, directed onto the backside 82 of the wafer 36. Inthe examplary embodiment, the wafer 36 is reduced from a thickness ofabout 32 mils to about 7±0.5 mils. The protective tape 52 protects thefrontside 40 of the wafer 36 and also acts as a cushion to absorb apressing force exerted by the rough grinding tool 90 during the grindingoperation. However, one problem resulting from the use of the protectivetape 52 is that an electrostatic charge may accumulate on the protectivetape 52 during the grinding operation.

After the rough grinding is completed, the wafer 36 is transferred fromthe rough grinding station 68 to the finish grinding station 70 by atransfer arm 71. The wafer 36 is secured to a vacuum chuck table 92. Afine grinding media is dispensed and a finish grinding tool 94 isdirected onto the backside 82 of the wafer 36 to remove defects such asscratches formed during the rough grinding operation. As such, thethickness of the wafer 36 is reduced primarily during the rough grindingoperation while the finish grinding operation simply polishes thebackside 82. Similarly, a further accumulation of electrostatic chargedmay be formed during the finish grinding operation.

After the finish grinding is completed, the wafer 36 is transferred fromthe finish grinding station 70 to the post cleaning station 72 by atransfer arm 96. The wafer 36 is secured to a vacuum chuck table 98, anddeionized water and a scrubber 100 are directed to the backside 82 ofthe wafer 36 to remove the residual grinding media. The wafer 36 is thenspin dried with nitrogen air. To further clean the wafer 36, a transferarm 102 transfers the wafer 36 from the post cleaning station 72 to thefinal cleaning station 74 where the wafer 36 is secured to a vacuumchuck table 104 and rinsed with deionized water and spin dried withnitrogen air.

The wafer 36 is then treated with ionized air during the transfer fromthe final cleaning station 74 to the unloading station 76. An unloaderarm 106 removes the wafer 36 from the vacuum chuck table 104 of thefinal cleaning station 74 and moves the wafer 36 to at an interimlocation within the unloading station 76. While at the interim position,ionized air is directed towards the wafer 36 to neutralize theaccumulation electrostatic charge. The ionized air may be provided by anair ionizing source 108 such as a Model A-300 manufactured by SimcoAerostat. The air ionizing source 108 is an electrically powered staticeliminator that blows ionized air to neutralize static charges onmaterials. An electronic balancing circuit 110 is provided to controlthe ion output ratio of negative-to-positive ions. Typically, theelectronic balancing circuit 110 is set to produce an ion output with anequal number of negative and positive ions. A control 112 on a frontpanel provides adjustment of the fan speed. The ionized air is directedtowards the interim location by a duct 114 coupling an output vent 116of the air ionizing source 108 with the unloading station 76. By usingan ESD meter, it has been observed that a static charge of about 4 to 6Kvolts is typically accumulated at the wafer 36 after completion of thefinish grinding operation. After subjecting the wafer 36 with ionizedair for approximately 5-10 minutes, the static charge is reduced toapproximately 0.2 to 0.4 Kvolts. Of course, an air ionizer with agreater ion output may be provided to shorten the neutralizing time.With the wafer 36 transformed from a warped state to a flat state, theunloading arm 106 feeds the wafer 36 into a carrier 118 located at theunloading station 76.

With respect to the backgrinding apparatus 28, the operation describedabove is repeated for processing subsequent wafers 36. It is noted thata wafer 36 is located at each station 64, 66, 68, 70, 72, 74, 76 duringthe operation of the backgrinding apparatus 28. In other words, thefollowing operations are performed simultaneously: a first wafer 36 isunloaded from the carrier 62 at the unloading station 64, a second wafer36 is cleaned at the precleaning station 66, a third wafer 36 is groundat the rough grinding station 68, a fourth wafer 36 is ground at thefinish grinding station 70, a fifth wafer 36 is cleaned at a postcleaning station 72, a sixth wafer 36 is cleaned at the final cleaningstation 74, and a seventh wafer 36 is neutralized and loaded into acarrier 118 at the unloading station 76. After the wafer 36 is loadedinto the carrier 118 at the unloading station 76, the carrier 118 isindexed such that an empty slot is available to accept the next wafer36. Since each of the wafers 36 are flat as a result of neutralizing theaccumulation of electrostatic charge, the unloading arm 106 is able toload the wafer 36 into the carrier 118 without difficulty. Inparticular, there is sufficient clearance for the unloading arm 106 tofeed the wafer 36 into the carrier 118.

Referring to FIG. 6, the protective tape 52 from each wafer 36 isremoved at the detapping apparatus 30. The detapping apparatus 30comprises a loading station 120, a tape removing station 122, and anunloading station 124. The operator transfers the carrier 118 from theunloading station 76 of the backgrinding apparatus 28 to the loadingstation 120 of the detapping apparatus 30. A transfer arm 126 unloadsthe wafer 36 from the carrier 118, and transfers the wafer 36 to a chuck128 located at the tape removing station 122. An uncoiler 130 dispensesa peeling tape 132 from a coil 134 and applies the peeling tape 132 ontothe protective tape 52. A roller 136 presses the peeling tape 132 ontothe protective tape 52 to further bond the peeling tape 132 to theprotective tape 52. By heating the chuck 128, an adhesive layer of theprotective tape 52 is softened. The peeling tape 132 is then recoiledonto the coil 134. As the peeling tape 132 is recoiled, the protectivetape 52 is peeled away from the frontside 40 of the wafer 36 because theprotective tape 52 remains bonded to the peeling tape 132 during therecoiling operation. With the completion of the detapping operation, thewafer 36 is transferred from the tape removing station 122 to theunloading station 124, wherein a transfer arm 134 loads the wafer 36into a carrier 136 located at the unloading station 124. The remainingwafers 36 are processed in a similar fashion.

Referring to FIG. 7, a dicing tape 138 is applied to the backside 82 ofthe wafer 36 by the dicing tape applying apparatus 32. The dicing tapeapplying apparatus 32 comprises a loading station 140, dicing tapeapplying station 142, and an unloading station 144. The operatortransfers the carrier 136 from the unloading station 124 of thedetapping apparatus 30 to the loading station 140 of the dicing tapeapplying apparatus 32. A transfer arm 148 transfers the wafer 36 fromthe loading station 140 to the dicing tape applying station 140. At thedicing tape applying station 140, the wafer 36 is applied to the dicingtape 138 which is spread on a wafer frame 152 so that the frontside 40of the wafer 36 faces upwardly. The dicing tape 138 is formed of aresin, and a pressure sensitive adhesive is applied to the surface ofthe dicing tape 138. The assembly, which comprises the wafer frame 152,dicing tape 138, and wafer 36, is then transferred from the dicing tapeapplying station 142 to a carrier 154 at the unloading station 144 by atransfer arm 146. The remaining wafers 36 are processed in a similarfashion.

Referring to FIG. 8, the wafer 36 is diced by the wafer dicing apparatus34 wherein the wafer 36 is diced by a dicing saw 156. The dicing isperformed while monitoring an image of a scribe line formed on thefrontside 40 of the wafer 36. Thereby, the wafer 36 is divided into aplurality of semicondcutor chips.

FIG. 9 illustrates another exemplary embodiment of a backgrindingapparatus 158. The backgrinding apparatus 158 is similar to thebackgrinding apparatus 28 illustrated in FIG. 5 with the exception thatan air ionizing source 160 is located within an unloading station 162.As such, similar elements are identified with the same referencenumeral. The Model A-300 air ionizing source may be used or any othersource which may fit within the unloading station. The air ionizingsource 160 is positioned so that an output vent 164 directs the ionizedair towards the interim position of the transfer arm 106. As such, aduct is not required. After the background wafers 36 are neutralized,they may be processed in accordance with the processes described inFIGS. 6-8.

FIG. 10 illustrates another exemplary embodiment in which the warpedwafers 36 may be flattened by neutralizing the accumulation ofelectrostatic charge. The backgrinding apparatus is similar to thebackgrinding apparatus 28 illustrated in FIG. 5 with the exception thatan air ionizing source is not provided. In this application, thebackground wafers 36 are sufficiently flat such that it is not necessaryto neutralize the accumulation of electrostatic charge prior to postcleaning station, final cleaning station, and unloading. After thewafers 36 are processed through the backgrinding apparatus, a carrier163 is transferred to an air ionizing apparatus 164 to flatten thewafers 36. The air ionizing apparatus 164 comprises a table 166, an airionizing source 168 such as the Model A-300 positioned on the table 166,and a receiving member 170 positioned on the table 166 and approximately8 to 12 inches from the air ionizing source 168. The carrier 163 isplaced on the receiving member 170, and the air ionizing source 168 isactivated. An output vent 172 of the air ionizing source 168 directs theionized air towards the carrier 163 and background wafers 36. Generally,the background wafers 36 are subjected to the ionized air forapproximately 5-10 minutes to adequately neutralize the accumulation ofstatic charge. Since the entire batch of background wafers 36 aresimultaneously neutralized, this exemplary embodiment is particularlyadvantageous for systems requiring a relatively high process throughput.After the background 36 wafers are sufficiently neutralized, they may beprocessed by the detapping apparatus 30, dicing tape applying apparatus32, and wafer dicing apparatus 34.

In certain applications, air ionization may not be required prior topost cleaning and final cleaning. However, the background wafers 36 maynot be sufficiently flat for subsequent handling at the unloadingstation of the backgrinding apparatus. In this situation, the backgroundwafers 36 may be loaded onto the carrier 163 at the unloading station ofthe backgrinding apparatus without electrostatic charge neutralizationif half the slots of the carrier 163 are loaded with the backgroundwafers 36. In other words, an empty slot is provided between eachbackground wafer 36 to provide sufficient clearance during the loadingand unloading process. The carrier 163 may then be processed at the airionizing apparatus 164 to neutralize the accumulation of electrostaticcharge. After the background wafers 36 are sufficiently neutralized,they may be processed by the detapping apparatus 30, dicing tapeapplying apparatus 32, and wafer dicing apparatus 34.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Thespecification and drawings are, accordingly, to be regarded in anillustrative manner rather than a restrictive sense.

1. A method comprising: directing ionized air at a substrate to reducesubstrate warpage, the ionized air reducing an accumulation ofelectrostatic charge.
 2. The method of claim 1, further comprising:applying a protective tape on a frontside of the substrate before saiddirecting ionized air at the substrate; and grinding a backside of thesubstrate before said directing ionized air at the substrate.
 3. Themethod of claim 2, further comprising: forming a circuit pattern on thefrontside of the substrate before said applying a protective tape on thefrontside of the substrate.
 4. The method of claim 3, wherein thesubstrate is a semiconductor wafer.
 5. The method of claim 2, whereinsaid applying the protective tape on the frontside of the substratecomprises: laminating the protective tape to the front surface of thesubstrate; cutting the protective tape along a contour of a substrateedge; and roller pressing the protective tape onto the frontside of thesubstrate.
 6. The method of claim 2, wherein said grinding the backsideof the substrate comprises: cleaning the substrate; rough grinding thebackside of the substrate at a first grinding station; finish grindingthe backside of the substrate at a second grinding station; and cleaningthe substrate.
 7. The method of claim 2, wherein said directing ionizedair at the substrate comprises: loading the substrate onto a carrierafter said grinding the backside of the substrate; and simultaneouslydirecting negatively and positively charged air ions onto the substrateand protective tape to reduce an accumulation of electrostatic chargeresulting from said grinding the backside of the substrate.
 8. Themethod of claim 7, wherein the negatively and positively charged airions are simultaneously directed onto the substrate and protective tapefor 5 to 10 minutes to decrease the accumulation of electrostatic chargedecreases from approximately 4 to 6 Kvolts to approximately 0.2 to 0.4Kvolts.
 9. The method of claim 2, wherein said directing ionized air atthe substrate comprises: directing ionized air at the substrate prior toloading the substrate into a carrier. 10-27. (canceled)
 28. A methodcomprising: reducing a thickness of a substrate; and directing ionizedair onto the substrate prior to dicing of the substrate and after thethickness of the substrate is reduced by the grinder, the ionized airreducing an accumulation of electrostatic charge on the substrate toreduce substrate warpage.
 29. The method of claim 28, wherein thesubstrate is a semiconductor wafer with a frontside of the semiconductorwafer having a circuit pattern.
 30. The method of claim 28, whereinreducing the thickness of the substrate includes grinding a backside ofthe semiconductor wafer.
 31. The method of claim 28 further comprising:receiving a first carrier loaded with a plurality of the substratesincluding the substrate; cleaning the substrate prior to grinding;cleaning the substrate after grinding; and loading the substrate into asecond carrier.
 32. The method of claim 31, wherein the ionized air isdirected onto the substrate prior to loading the substrate into thesecond carrier.
 33. The method of claim 31, wherein the ionized air isdirected onto the substrate after loading the substrate into the secondcarrier.
 34. The method of claim 28, wherein prior to directing theionized air, the method further comprises covering a surface of thesubstrate with a protective tape, and wherein the ionized air reduces anaccumulation of electrostatic charge on the protective tape to reducesubstrate warpage.